A pivoting power transmission unit of the type presented above is described in particular in patent application FR 01 14392, in an application fitted to a convertible aircraft with tilting rotors, in particular as described in French patents FR 2 791 319 and FR 2 791 634 to which reference should be made for further details.
It is pointed out however that these two patents describe a convertible aircraft with tilting rotors, of the type in which the shaft of each rotor (which is a drive shaft projecting from a pivoting power transmission unit) is driven in rotation about itself by a front reduction gear unit of one respectively of two power transmissions each also comprising a rear reduction gear unit linked to the corresponding front reduction gear unit, and also to one respectively of two engines (turboshafts), each supported by one respectively of the two fixed wings of the aircraft, and to an interconnecting shaft linking the two transmissions for the two rotors to be driven in rotation by any one of the two engines in case of failure of the other engine.
The drive shaft of each rotor, the corresponding power transmission and the corresponding engine are housed in one respectively of two articulated pods, each comprising a front part mounted, so as to pivot about the pivot axis, on a rear part stationary relative to the aircraft, and attached to one respectively of the two wings of the aircraft, and in which are housed the corresponding engine and at least in part the rear reduction gear unit of the corresponding transmission, the front reduction gear unit of which, embodied as a helicopter main gearbox, and also the shaft of the corresponding rotor are housed in the pivoting front part of the pod, and are mounted so as to pivot with this front part relative to the stationary rear part of the pod and the corresponding fixed wing.
On a helicopter, it is known practice to absorb main rotor lift by a set of suspension struts, suspending on the structure of the helicopter a main gearbox mounted between the engine or engines driving the main rotor and the latter, this set of suspension struts comprising at least three and generally four rigid struts, substantially rectilinear, distributed around the main gearbox and inclined towards each other and towards the axis of rotation of the rotor shaft at their upper ends at which the suspension struts are articulated to the upper part of the main gearbox casing, which also comprises a lower part of the casing attached to the upper part of the casing, while the suspension struts are integral with the structure of the helicopter at their lower ends, articulated on fittings integral with the transmission support platform of the structure.
The upper and lower parts of the casing are rigidly assembled to each other by a ring of threaded studs which at the same time secure to the casing an outer peripheral gearwheel of an epicyclic train, constituting an output stage of a pivoting reduction gear unit housed in the casing, and such that the planet gear carrier of the epicyclic train is integral in rotation, about the axis of rotation of the rotor, with the drive shaft mounted rotatably in the upper part of the casing by rolling bearings, including a thrust bearing axially retaining the drive shaft in the upper part of the casing, while the sun gear of the epicyclic train is integral in rotation with a bevel or spiral bevel gear meshed with a bevel or spiral bevel input gear of a pair of bevel gear meshed with a bevel or spiral bevel input gear of a pair of bevel gears, constituting another reduction stage housed in the lower part of the casing, and such that the bevel input gear shaft is driven from the engine or engines.
In an architecture of this type, all the loads and bending moments applied to the top of the shaft driving the rotor pass through all of the gears and other toothed components housed in the casing. The deformation of the casing under these loads must therefore be precisely known, in order that the bearing surfaces and contact areas on the bevel or spiral bevel gears can be optimised.
On a helicopter, this problem can be solved, since the main rotor always occupies the same position.
On the other hand, on a convertible aircraft, each tilting rotor can assume a large number of different positions about the pivot axis, ranging between the aeroplane and helicopter configurations. To each position there corresponds a type of deformation of the toothed components of the transmission stages housed in the casing, and this makes it all the more complicated to optimise the contact areas of these toothed components, and in particular of the bevel or spiral bevel gears.
This disadvantage is encountered on the pivoting power transmission units according to the French patent application and the two French patents cited above.